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Stars In An Aged Universe
For a classification of stars see The H-R Diagram. This article is about the kind of stars that will exist far in the future. Such stars currently don't exist. However, according to some theoretical models, time might not flow with the same speed and there might be places in the Universe that are very old. Also, according to the multiverse theory, there might be much older universes then ours. Nevertheless, an advanced civilization, able to perform time travel, might be able to send settlers further in time. Matter circuit in nature There is an endless exchange of matter between interstellar environment and celestial bodies. Stars and planets are formed from interstellar clouds of gas and dust. Black holes and giant planets swallow matter continuously. On the other hand, stars and small planets lose matter continuously, as solar winds and eroded atmospheres. When they die, giant stars lose a significant part of matter into the interstellar environment, as they go tot the red giant phase or they end-up in a supernova. Overall, there are two major differences that occur in nature: # Matter from celestial bodies (stars, planets or stellar remnants) don't return completely to the interstellar environment. # Matter lost by celestial bodies has a different composition. As one can see, the interstellar environment is slowly depleted of matter. As stars are born, not all their matter returns into interstellar environment. A significant part remains as stellar remnants (black holes, neutron stars, white dwarfs). With less matter in circulation, star formation becomes more and more rare. Also, interstellar environment changes composition. The Primordial Stars were formed of hydrogen with traces of helium. With each supernova, a significant amount of hydrogen is transformed in heavier elements. In any interstellar cloud of gas and dust, there is a fight between two forces. Gravity tries to force the cloud collapse, while pressure tries to make the cloud expand. When the cloud is hotter, pressure is also higher. The minimal mass a cloud requires to collapse is known as the Jeans mass. With less stars around, temperature decreases. Also, as a cloud collapses, it generates heat. Hydrogen and helium radiate less heat as infrared light then heavier elements. A cloud with higher metalicity will radiate more infrared and will cool faster. This will cause the Jeans mass to decrease, allowing smaller clumps of gas to collapse. Future stars will contain a higher amount of heavier elements, but will still have enough hydrogen to sustain fusion. The formation of large stars, able to fuse heavier elements then helium, will cease far before hydrogen will be exhausted. Then, there will come a time when very small clouds of gas will collapse, rarely having enough mass to sustain fusion. Still, by that time, some stars that are still shining today, will be shining too. Very Old Stars There are a few stars that exist today and that will still shine when the last stars in the Universe will be formed: M - type stars, also known as red dwarfs, are very dim and fuse their hydrogen slowly. Most of them will still be shining. Blue Dwarfs are a hypothetical type of stars. Near the end of their lifetime, M - type stars accelerate the rate of hydrogen fusion, increasing temperature and becoming blue, before ending-up as helium white dwarfs. White dwarfs are stellar remnants. They don't support any kind of fusion. Because they are very compact and radiate their heat very slow, they might still be warm enough to shine. Black Dwarfs are cooled white dwarfs, which had radiated nearly all of their heat into space. Neutron stars might survive that long or might not. The subject is still under debate. Some neutron stars collapse into black holes, while others might explode. Black holes are expected, near the end of their lifetimes, to radiate heat and light as Hawking radiation. They might be the last objects glowing in space. This theory is not fully accepted by the science community. Iron stars are expected to form in theory, if protons don't decay. The idea is that inside stellar remnants cold fusion still continues and slowly matter is fused into iron, which is the element with the strongest binding energy. However, this process, even if it requires a very long time, produces only little energy. The Last Formed Stars Frozen stars are expected to form in an environment with high metalicity. Heavier elements form an insulation layer around the stellar core. These stars will not lose their internal energy fast. In these conditions, hydrogen fusion can occur even at lower masses, which current only allow deuterium fusion, like in Brown Dwarfs. These stars will radiate far less energy then current stars. Their surface temperature will be a lot lower, even below water freezing point. So, in their upper layers, we could see clouds of water or even water ice. Because they will radiate energy much slower, they will also live much longer then the stars that exist today. Stars with little hydrogen. The last stars to be formed might have a composition like this one: hydrogen 5%, helium 55%, heavier elements 40%. Hydrogen fusion will only last for a short amount of time, despite the thermal insulation that heavier elements will provide. Also, heavier elements tend to migrate to the core, while lighter elements migrate to upper layers. So, hydrogen fusion will not last long. If the star is large enough, it will behave like a sub-giant star and fast will evolve towards a red giant. Helium fusion will start fast. The main lifetime of the star will be dominated by the red giant phase, which is a terminal phase for stars in our epoch. Metal stars. In Astronomy, the term metals refer to all elements other then hydrogen and helium inside a star. So, metal stars will be stars composed mostly of heavier elements and not of metals. The last stars formed in the Universe will have little hydrogen and maybe also little helium to fuse. What would happen if a star contains 2% hydrogen, 8% helium and 90% metals? Classic hydrogen and triple alpha fusion processes will not be able to sustain the star for long. In theory, such stars will never have a chance to form, because by that time the Jeans mass will be very low. Instead, what would form would actually be planets. If somehow a star is formed, it will only have little mass and will fast end-up as a white dwarf. Still, if somehow a heavier celestial body is made, able to fuse something heavier, it will become a short lived star. Carbon fusion cannot sustain a star for more then 1000 years, while heavier elements (like neon or oxygen) cannot sustain a star for more then a few years. In the end, it will explode as a supernova. Collision stars. This is how the last stars might be formed: through collisions. This way of forming stars is proven by many scientists, but also by amateurs using Universe Sandbox and other simulators. The ways of doing this are as follows: # When a few giant planets collide, the resulting object might have enough mass to become a brown dwarf or even a red dwarf star. # Exhausted brown dwarf (Y spectral class) can collide to form a red dwarf. # Hydrogen white dwarfs (expected to form from former large brown dwarfs) might collide and reach enough mass to tart hydrogen fusion and to become a red dwarf. # Helium white dwarfs (resulted from cooled red dwarfs) can collide and initiate helium fusion. If they don't end-up in a supernova, they can become red giants and fuse helium. # Classic white dwarfs (composed of carbon and oxygen) might collide. There is a high chance that this will end-up in a supernova. However, if they survive and achieve enough mass, they can start carbon fusion and survive for up to 1000 years. # High mass white dwarfs can accrete extra mass. When they exceed 1.4 Solar masses, they will collapse into a neutron star, also producing a supernova. However, the neutron star will be hot and will last long. It is possible that the last civilizations living at that time will try these methods to produce more energy for themselves. In addition, an advanced civilization will try to create an artificial quasar. If matter is allowed to fall in a black hole in a controlled way, we can produce energy to illuminate a planet and make it habitable. Planets At that time, the Universe will have plenty of planets. However, around stars, there will not be many planets, because of long-term orbital perturbations. In the last stages, collapsing molecular clouds will most often create planets then stars. However, on these planets, conditions will be far different. With little hydrogen, there will be less water and less methane. Also, radioactive elements like uranium or thorium will no longer exist. Without massive supernovas, there will be no force able to create them. Old planets who survived will be depleted of water and volatiles. Their surface will be perfectly flat. In these conditions, life will be hard for Earth-like organisms and for humans. If the same kind of humans will live to those days, they will find very hard to find a source of energy. Even finding water will be hard, they will have to look at hydrated minerals buried deep underground. The sky will be black, with rare, artificial stars. Category:Stars and other hosting celestial bodies